1. Field of the Invention
The present invention relates to the compensation of a video image on one video monitor to provide an accurate reproduction of the tonal luminance and tonal luminance difference ratios and color of the image on another video monitor.
2. Description of the Prior Art
At the present time, in a number of fields, a video image on one video monitor is part of the image reproduction chain, and it is desired that a video image on another video monitor be an accurate reproduction of the tonal luminance differences of the first image. For example, in the medical field a CAT X-ray scan, an ultrasonic scan, an NMR scan (nuclear magnetic resonance) or thermograph image is produced on a video monitor CRT (cathode ray tube) screen for immediate viewing by the technical personnel operating the instrument. Simultaneously the same image is produced on another video CRT screen for view by the physician.
In the printing field it is sometimes desired to make an accurate printed picture in various cities or at different times from the image on a video monitor screen. For example, the screen may show a computer-generator image. That image is reproduced on a photosensitive material or directly on a printing plate to print a hard copy that should be similar in tone, luminance and color to the original object or scene. It may also be desired that the image reproduced by a VCR be an accurate reproduction of the original real time image.
It has been found that video image reproduction may vary significantly from the original image on the video monitor screen. Some of the distortions are due to the inaccuracy of the CRT screen and video image reproduction process. That type of distortion has been recognized and compensation methods have been suggested, generally dealing with the problem as it affects an entire group of video monitors. Similarly, for hard copy reproduction the distortions due to the camera have been recognized and treated, generally by improvements in the optics of the camera or overall corrections in video components (brightness, contrast, etc.). However, many of the distortions are not predictable and may vary form day to day and from one monitor or VCR to another.
Set forth below is a discussion of the problems most frequently encountered in producing an accurate picture in the video-to-video process as it relates to accurate tonal black-white reproduction. The contribution of each problem to the total final distortion of the image can change in its characteristics periodically and is not predictable. One-day the CRT power may be incorrect and seriously distort the picture, and the next day it may still be incorrect but have only a minor adverse effect due to partial compensation distortions from other components in the system. In addition, most of the problem-causing effects are non-linear, so that complete compensating for them in a simple direct way is impossible.
The problems with the conventional system are explained in connection with FIG. 1A which is a block diagram of a conventional black-white video system. As shown in FIG. 1A, the video image is produced by the video source 10, which may be a video camera, a computer graphics output, or a VCR. The video signal is viewed directly on the first monitor CRT screen 11. The same video image is shown on a second CRT screen 13 of the second monitor 14. Generally the image on screen 13 is a positive image compared to the image on monitor screen 11.
Each step of this conventional process gives rise to unpredictable distortions. The first set of distortions arises in the CRT device, and its screen 13, which is part of the monitor 14. The ratio between luminance values, i.e., the ratio between shades of gray, on the screen 13 and/or the screen 11 may be different. For example, either or both CRT tubes may be unevenly coated with phosphor, or may be aged or may be subject to glare. In addition, the relationship of the signal voltage applied to produce a certain brightness is not linear. Consequently, the small differences in voltage may give rise to relatively large differences in brightness. A detailed description of the inaccuracy of a CRT distortion in an electronic camera, is found in Schwenker, R. P., "Film Selection Considerations For Computed Tomography and Ultrasound Video Photography": Proc. SPIE - Appl. of Optical Instrumentation In Medicine, VII, 1979; 173, pgs. 75-80.
The present invention is particularly directed to accurate reproduction of the luminance differences in value (differences in a gray scale) and absolute luminance on a black-white video screen. However, in its broader aspects, the invention is also applicable to the accurate reproduction of color images. The invention is directly applicable to color images in the sense that the video screen may be a color CRT screen and the invention will correct for gray scale distortions in reproducing the image on the color screen. In addition, the reproduction of color images has its own set of problems and distortions, aside from black-and-white tonal differences. These color distortions can also be corrected, and their correction will be discussed at the end of the detailed description.
These color and luminance distortions include that the original object color is not exactly matched to the phosphors on the CRT screen so that the color on the screen does not match the original object color. A further problem with color, not found in black-white images, is that the perception of color of the object or video screen (by the human eye) differs from the actual color.
In the U.S. Pat. No. 4,263,001 entitled "Apparatus and Method For Enhancement of Optical Images", in one embodiment, which is not claimed, a video camera is connected to an electronic image modification divide which, in turn, is connected to a single frame storage, to prevent feedback, and a monitor CRT.
In U.S. Pat. Nos. 4,492,987 and 4,520,403 both entitled "Processor For Enhancing Video Signals For Photographic Reproduction", the screen of an electronic camera is electronically modified to enhance photographic reproduction. The entire screen is treated as a unit and its brightness or color is changed in accordance with the distortion introduced by a selected photographic film.
In U.S. Pat. No. 4,658,286 entitled "Method and Apparatus For Correcting Distortions in Reproducing Systems", a type of feedback system is described. In one embodiment three photocells look at a corner of the CRT screen having test colors and their outputs are compared to reference colors.